SUBARU FORESTER ENGINE PROBLEM

1998 Subaru Forester • 200,000 miles

When engine is cold car starts for a second then stalls. Gradually stays running longer till it stays running after 15 to 20 min of trying. Codes po301 runs fine after starting. If I blow heat under engine compartment for 30min or so it starts right up. Have changed coil pack, coolant temp sensor, cam sensor, fuel filter, plugs, still same